1. Field of the Invention
The present invention relates to a solar battery cell and a method for manufacturing the same, and more particularly to a solar battery cell having a non-reflective surface configuration (hereafter referred to as "textures") and a method for manufacturing the same.
2. Description of the Related Art
FIG. 8 shows an example of a cell cross-section of a conventional solar battery cell. This solar battery cell 1.sub.0 is generally referred to as NRS/BSF (Non-Reflective Surface and Back Surface Field) type solar battery cell, and comprises a P-type silicon substrate 4 whose light-receiving surface includes an N.sup.+ -type diffusion layer 3 formed by thermal diffusion of N-type impurity ions so as to effectively take in carriers generated by light energy, and includes uneven textures (texture pieces) 8 having an inverted-pyramid shape formed so as to reduce surface reflection. An oxide film layer 7 is formed on the N.sup.+ -type diffusion layer 3 so as to reduce recombination of the carriers in the surface. Also, in an opening where the oxide film layer 7 is not formed, a surface electrode 2 is formed in a comb-like shape for effectively taking out the generated electricity and is directly connected to the N.sup.+ -type diffusion layer 3. Further, the oxide film layer 7 is covered with an anti-reflection film 10 for reducing the surface reflection of incident light.
A back surface of the P-type silicon substrate 4 includes a P.sup.+ -type diffusion layer 5 formed by thermal diffusion of P-type impurity ions so as to increase the amount of carriers generated by light energy. An oxide film layer 7 for reducing recombination of the carriers and a back surface electrode 6 for reflecting a long-wavelength light which may escape away from the back surface and for taking out the generated electricity are formed almost over an entire surface of the P.sup.+ -type diffusion layer 5. The P.sup.+ -type diffusion layer 5 and the back surface electrode 6 are connected via an opening (not shown) formed in the oxide film layer 7.
In the solar battery cell 1.sub.0 of NRS/BSF type such as shown in FIG. 8, the textures 8 formed on the light-receiving surface serves to allow multiple reflection of incident light so as to increase the amount of light that reaches the inside of the cell. Therefore, the size, the area of the formed textures, the configuration of the textures and the like have a great influence on an output power by changing the energy conversion efficiency of the generated electricity. Accordingly, how the size and the shape of these textures 8 are to be configured is an extremely important factor in manufacturing the solar battery cell.
In other words, the incident light undergoes multiple reflection on the light-receiving surface due to these textures 8, as shown in FIG. 9, whereby the surface reflectivity is reduced. As a result of this, the amount of light absorbed by the substrate 4 increases, whereby more electric currents are generated. Especially, in the case of a solar battery cell for use in space, it receives radioactive rays (cosmic rays). In this case, if the textures 8 are present on the substrate 4, the incident light is refracted at the light-receiving surface to be incident in an oblique direction into the substrate, whereby the carriers generated near a PN-junction formed about the surface of the solar battery cell increase in number and the influence on the carrier lifetime by deterioration due to the radioactive rays can be reduced.
Accordingly, to form a larger texture area on the light-receiving surface is effective in improving the output of the solar battery cell 1.sub.0.
Therefore, the conventional solar battery cell 1.sub.0 has a configuration of textures 8 such as shown in FIGS. 10A, 10B, 10C, and 11. Here, FIG. 10A is a plan view showing an entire solar battery cell. FIG. 10B is a partially enlarged view of FIG. 10A. FIG. 10C is an enlarged plan view showing a portion indicated by symbol B in FIG. 10A. FIG. 11 is an enlarged plan view showing a portion indicated by symbol D in FIG. 10C. In these Figures, a solar battery cell 10, a grid electrode 2, a bar electrode 3, and a connector (pad) electrode 4 for taking out an output power are shown.
The conventional solar battery cell la has a structure in which the textures 8 are arranged both in longitudinal and lateral directions (in a checkered configuration) in all the regions except for a site where the surface electrode 2 is formed. The textures 8 formed at the sites other than the outer perimeter 18 have an uneven shape of inverted-pyramid type (having a square shape in a plan view) as shown in FIG. 11, and all have the same size and the same shape. Also, all the distances d between adjacent textures 8 are the same. Further, textures 8 are also formed on the outer perimeter 18 of the solar battery cell 1.sub.0, as shown in FIG. 10B.
Hitherto, in order to form the textures 8 having a shape such as shown in FIGS. 8 to 11, the steps shown in FIGS. 12A to 12G, for example, are carried out.
First, referring to FIG. 12A, a silicon substrate 4 is prepared. Then, referring to FIG. 12B, an oxide film layer 7 is formed on a surface of the silicon substrate 4 by thermal oxidation or CVD. Subsequently, referring to FIG. 12C, a resist 15 is applied on the oxide film layer 7. Then, referring to FIG. 12D, a region having a predetermined texture pattern on a light-receiving surface is exposed to light and developed. By this process, a texture pattern is formed on the oxide film layer 7 by means of the resist 15. Subsequently, referring to FIG. 12E, an unnecessary portion of the oxide film layer 7 is removed by etching and then the resist 15 is removed. By this process, a texture pattern is formed on the silicon substrate 4 by means of the oxide film layer 7. Referring to FIG. 12F, the silicon substrate 4 in this state is etched for a predetermined period of time by using an etchant liquid having a predetermined temperature and a predetermined concentration, such as a high-temperature alkali solution. In the case of the silicon substrate 4, each crystal face has a different ratio of corrosion by a chemical agent. Fine inverted-pyramid type textures 8 can be formed by an anisotropic etching treatment using this property. Finally, referring to FIG. 12G, the oxide film layer 7 is removed to complete the textures 8 on the light-receiving surface of the silicon substrate 4.
However, conventional solar battery cells have problems such as indicated by the following (1) and (2).
(1) As described above, each texture 8 formed on the silicon substrate 4 at the sites other than the outer cell perimeter in a conventional solar battery cell has the same size and the same shape, as shown in FIG. 11, and moreover all the distances d between adjacent textures 8 are the same. PA1 (2) Also, in the conventional solar battery cell 1.sub.0, textures are also formed on the outer perimeter 18 of the solar battery cell 1.sub.0, as shown in FIG. 10B. Therefore, cracks or possibly cleavages are generated on the outer perimeter 18 of the cell 1.sub.0 in cutting out the cells in a dicing step even during the manufacture of the solar battery cell 1.sub.0. This makes it difficult to handle the solar battery cell 1.sub.0 and leads to decrease in the productivity and contamination of the production line.
For this reason, the area of the region on the solar battery cell 1.sub.0 where the textures 8 are formed may be small. Namely, it is not possible to form a texture pattern having the same size around the electrode 2 on the region where the textures 8 are formed, so that the portion around the electrode 2 is made flat. As a result of this, the area occupied by the flat portion on the light-receiving surface of the cell will be large and, in accordance therewith, the effect of reducing the reflectivity will be small, leading to decrease in the output power of a solar battery cell 1.sub.0.